Conventionally, a frequency synthesizer is provided in radio equipment to up-convert modulated signals to generate transmission signals (i.e. radio signals) of a carrier frequency band.
An example of such a frequency synthesizer is disclosed in Patent Literature 1. Meanwhile, a frequency synthesizer is referred to as “transmitting apparatus” in Patent Literature 1 and therefore the apparatus disclosed in Patent Literature 1 will be referred to as “transmitting apparatus” below. FIG. 1 shows a schematic configuration of the transmitting apparatus disclosed in Patent Literature 1. As shown in FIG. 1, voltage control oscillator 1 (hereinafter “VCO 1”) that changes the output frequency according to input voltage, mixer 2 that down-converts the output F_out of VCO 1, phase comparator 3 that compares the phases of the output of the mixer and reference signal F_IF and outputs the phase difference (i.e. phase error), loop filter 4 that smoothes the output signal of phase comparator 3, and local signal generating circuit 5 that generates a local signal of mixer 2, are provided. A PLL (Phase-Locked Loop) circuit having a function of shifting a frequency in this way using a mixer is referred to as “offset PLL.”
Here, F_IF is, for example, a modulated signal of the IF (Intermediate Frequency) band generated in a quadrature modulator. F_IF is up-converted to the RF (Radio Frequency) band by PLL that controls VCO 1, and the up-converted transmission signal F_out of the RF band is outputted from VCO 1. The noise components of the modulated signal F_IF of the IF band are suppressed by frequency characteristics of PLL that controls VCO 1 and, consequently, the configuration shown in FIG. 1 is generally used popularly in GSM mobile telephones and so on.
Patent Literature 2 discloses a VCO frequency band switching-type frequency synthesizer that uses a PLL circuit. FIG. 2 shows the configuration of the frequency synthesizer disclosed in Patent Literature 2. As shown in FIG. 2, PLL section 11, frequency band selecting circuit 12 that outputs a selection signal S for selecting and shifting the oscillation frequency band of VCO 13 in response to a shift signal SS, sampling circuit 14 that generates a sampling value CO acquired by supplying a ground voltage GND to VCO 13 at the oscillation frequency band that is selected first and by sampling and counting a VCO output signal VO for a certain period, comparing circuit 15 that compares the sampling value CO and an expected value EX and outputs a voltage control switch signal SV and the shift signal SS according to the comparison result and expected value register 16 that holds the expected value EX, are provided.
FIG. 3 shows control voltage SC-versus-oscillation frequency characteristics in VCO 13 in FIG. 2. In case where VCO 13 is incorporated in an LSI, the distribution of element values makes the distribution of oscillation frequencies significant. Therefore, as shown in FIG. 3, VCO 13 is configured to cover a wide range of the oscillation frequency such that the oscillation frequency can be switched between a plurality of frequency bands A to G. Then, when the frequency of the output signal VO is switched, the output frequency from VCO 13 is measured in the counter circuit and an optimal frequency band is selected by frequency band selecting circuit 12 from a plurality of frequency bands shown in FIG. 3.
Citation List
Patent Literature
    PTL 1: Japanese Patent Application Laid-Open No. 2008-109680    PTL 2: Japanese Patent Application Laid-Open. No. 2001-251186    PTL 3: U.S. Pat. No. 6,269,135